(a) Field of the Invention
The present invention relates to a zoom lens system, and more specifically to a compact zoom lens system suited for use with compact cameras.
(b) Description of the Prior Art
In the recent years where cameras are automated and designed compact, it is strongly demanded to develop compact photographic lens systems. On the other hand, it is strongly needed to switch photographic lens systems between the wide position and the tele position even for simple cameras such as lens shutter cameras which do not permit exchange of photographic lens systems and it is practised accordingly to design photographic lens systems as zoom lens systems. However, zoom lens systems are usually larger and especially longer in total lengths thereof than lens systems designed for use at a single focal point. Therefore, adoption of zoom lens systems is conflicting with the demand for compact cameras.
As zoom lens systems having total lengths as short as possible and compatible with compact cameras, there have already been proposed so-called telephoto type zoom lens systems each consisting of two lens units, i.e., a front lens unit having positive refractive power and a rear lens unit having negative refractive power. So far as zooming ratios are within a range of up to 2, it is possible to design the zoom lens systems of this type so as to be compact and have high performance. The zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 264019/62, for example, is a zoom lens system consisting of eight lens elements and having high performance owing to the fact that the zoom lens system adopts an aspherical surface.
When an attempt is made to design a more compact zoom lens system, however, there is posed a problem. Speaking concretely, it is generally necessary to strengthen the refractive power of each lens element by shortening the radius of curvature on each lens surface in order to shorten the total length of a zoom lens system. However, the shift of a lens element having strong refractive power results in remarkable variation of aberrations, thereby degrading imaging performance of the zoom lens system. In order to reduce the variation of aberrations, it is necessary to use a large number of lens elements, thereby making it impossible to shorten total length of the zoom lens system.
In order to solve this problem, it is conceivable to use the so-called graded refractive index lens (GRIN lens). For example, Japanese Unexamined Published Patent Application No. 259216/61 discloses a telephoto type zoom lens system consisting of two lens units in which number of lens elements are made very small by using a GRIN lens having refractive index gradient in the direction along the optical axis (so-called axial GRIN lens) and/or a GRIN lens having refractive index gradient in the direction perpendicular to the optical axis (radial GRIN lens). However, this zoom lens system has a low zooming ratio of 1.5 at best and a defect that it cannot provide a sufficient effect of the focal length change.